Burners, in particular premix burners, are normally equipped with an additional pilot burner in order to ensure stable combustion over a wide operating range, in particular at no-load and within the partial-load range.
In the case of the premix burner, a “double-cone burner”, disclosed by EP 0 321 809 A1, this pilot burner is realized by fuel being injected in the center of the cone. The gas flowing into the interior space of the double-cone burner burns in a diffusion flame stabilized deep within the interior space of the burner.
EP 0 704 657 A2 discloses a further premix burner in which the pilot burner is realized by fuel flowing from an annular gas duct having outlet holes inclined outward into the outer backflow zone of the combustion chamber downstream of the burner exit. The outflowing gas burns in a diffusion flame stabilized by the jump in cross section at the burner exit.
Both embodiments of burner and pilot burner disclosed by the abovementioned documents ensure stable combustion over a wide range of 10 to 100% pilot-gas proportions. However, these known systems also have some disadvantages.
Even small quantities of, for example, 10% pilot gas may lead to markedly increased pollutant emissions, since the flames work in diffusion operation. This is undesirable in particular during part-load operation. In order to achieve large extinction distances, pilot-gas proportions of up to 100% are necessary, which may lead to very high emission values within the starting range and the low load range.
In the embodiment of the internal piloting according to EP 0 321 809 A1, it is possible in certain designs for bimodal flame stabilization to occur during the switch-over operation from pilot to premix combustion. That is to say that the anchoring point of the flame is not clearly defined and varies dynamically between pilot flame stabilized in the burner and premix flame stabilized on the outside, which may lead to the excitation of thermoacoustic instabilities. In the embodiment of the external piloting according to EP 0 704 657 A2, the stabilizing of the pilot flames in annular combustion chambers may be adversely affected, since pronounced transverse flows may form in the outer recirculation zones in a multiburner arrangement.